The present invention relates to composites, and more particularly to composites including a plurality of at least three substrates and an adhesive structure disposed intermediate adjacent substrates for securing them together to form composites suitable for use in medical applications such as surgical gowns.
Unless otherwise indicated, the term "composite" as used herein and in the claims includes a first substrate, a second substrate, a third substrate and an adhesive structure disposed intermediate each pair of adjacent substrates for securing the first, second and third substrates together. In a broader sense, the term "composite" may include first, second and third substrates which are bound together directly (that is, without an intervening adhesive structure) but such materials are not to be considered part of the present invention since the substrates useful therein are typically severely limited and the techniques required in order to cause lamination or joinder of the substrates typically require a property of at least one of the substrates to be significantly modified. Such a composite may require heat, pressure or a combination thereof to be applied to the substrates in order to join them, as by applying one substrate in a hot, molten form to the other substrate or passing an assembly of the substrates through the heated nip of a pair of pressure rolls (i.e., calendering). In such cases, the heat and/or pressure tends to significantly modify the properties of at least one of the substrates--for example, converting a breathable or vapor-permeable substrate into a non-breathable or vapor-impermeable substrate. Such a calendering process limits the possible substrate combinations since all substrates should have a similar melting point in order to achieve adequate bonding without creating in the substrates heat-generated pinholes which might alter the properties thereof.
In a composite, as the term is defined above--that is, requiring in addition to the substrates an adhesive structure therebetween--there is less opportunity for one substrate to affect another substrate since the adhesive structure is disposed intermediate the two substrates and the two substrates are typically not in direct physical contact. On the other hand, the nature of the adhesive structure may itself affect the properties of either the substrates or the composite. Thus, the application of a hot melt adhesive to the substrates may cause melting of one or more substrates (since the hot melt adhesive is typically applied at temperatures of about 375.degree. F.) or the molten adhesive may flow into small apertures or pores of one of the substrates to render it less permeable. Even where the adhesive structure itself does not significantly alter the structure of the substrates directly, it may do so indirectly by modifying the properties of the composite. For example, if two breathable or water-permeable substrates are joined by a continuous adhesive structure which is non-breathable, the adhesive structure, in effect, negates the breathability of the substrates and results in a non-breathable composite.
Commonly assigned U.S. Pat. No. 5,342,469, the substance of which is incorporated herein by reference, teaches a wo substrate composite wherein an adhesive structure is disposed intermediate a pair of substrates for securing the substrates together to form the composite without significantly modifying the properties of either substrate directly. Furthermore, the adhesive structure does not significantly modify the properties of either of the substrates indirectly--that is, it does not modify the properties of the composite from what they would be if the composite consisted exclusively of the first and second substrates. As noted in the patent, the composites formed thereby have utility as medical composites for surgical gowns and the like because of their combined breathability (that is, high moisture vapor transmission rates) and resistance to penetration by liquids (which might carry bacteria). On the other hand, these two substrate composites are not satisfactory for use in medical applications such as surgical gowns and the like where there must be not only barrier resistance to penetration by liquids (that is, bacteria infiltration), but also having resistance to penetration by viruses (especially at elevated driving force pressures such as 3 psi) without sacrificing breathability.
Accordingly, it is an object of the present invention to provide a composite suitable for medical applications, including use as a surgical gown, where barrier resistance to vital penetration is required as measured by ASTM ES-22 (modified to a pressure of at least 3 psi).
Another object is to provide such a composite which, in preferred embodiments, passes the viral barrier test at a level of at least 6 psi.
It is a further object to provide such a composite which incorporates as a fabric substitute an adhesive-free liquid-resistant three layer non-woven formed by an internal meltblown fabric substrate disposed intermediate two spunbond fabric substrates and bonding them together.
It is also an object of the present invention to provide methods of making such composites.